The present invention relates to a tower furnace for heat treatment of metal strips having a pre-heating section and a high-temperature section connected thereto at the top forming a housing separate from the pre-heating section, whereby the pre-heating section equipped with a preferably gas-heated muffle exhibits a connecting stopper made of heat-insulating material for the high-temperature section inserted into the muffle.
Metal strips, in particular those made of chrome steel or chrome-nickel steel, are continuously bright-cooled in an inert gas atmosphere of hydrogen or a hydrogen-nitrogen mixture in so-called tower furnaces which can be designed with or without a muffle. The advantage of the heat treatment of metal strips over an externally heated muffle is that gas burners can be used without exhaust gases impairing the inert gas atmosphere in the muffle interior. The furnace temperature is, however, limited by the heat tolerance of the muffle. In addition, on account of the quantity of heat to be transferred from the muffle to the metal strip the length of the muffle determines the attainable throughput rate of the metal strip to be treated, such that the muffle clamped in the vicinity of its upper end is also subjected to load by the dead weight, apart from the heat load, which results in limiting of the length of the muffle as a result of the dead weight despite a wall thickness which increases from bottom to top in the longitudinal direction of the muffle. In this respect the stability of the special steel used for the muffle reduced at high temperature loads is to be considered.
Tower furnaces without muffles have a fireproof lining and are heated electrically so that higher furnace temperatures can be achieved. Since the fireproof furnace lining is somewhat porous when the tower furnace is opened it absorbs ambient air which has to be rinsed out by use of nitrogen gas, for example, before trouble-free heat treatment of the metal strips under inert gas atmosphere can be ensured. After they are opened tower furnaces without muffles are not ready for service again for several days on account of the required rinsing periods. There is also considerable hydrogen consumption, as the residue of atmospheric oxygen remaining after rinsing binds together with the hydrogen of the inert gas to form water.
In order to achieve high throughput of the metal strips to be treated, in spite of the restrictions associated with a muffle insert, it is known (EP 0 675 208 A1) to connect a high-temperature section without a muffle to the pre-heating section of a tower furnace, fitted with a muffle, which forms a housing separate from the pre-heating section, which is in turn set on the housing of the pre-heating section. The metal strip to be treated exits from the muffle of the pre-heating section through a connecting stopper made of heat-insulating material inserted into the top of the muffle into high-temperature section, where it is heated by means of the electrical heating to the required final temperature before it reaches a cooler set on the top of the tower furnace. Because of the high-temperature section without a muffle an advantageously lower outlet temperature for the muffle insert of the strip to be treated from the muffle can be taken into consideration so that the restrictions imposed by the heat and weight loads of the muffle cannot have an effect on the outcome of the heat treatment or the throughput rate. However, it is a disadvantage that the high-temperature section with its fireproof lining stipulates a rinse period which is considerably longer compared to muffle furnaces after the furnace is opened, with a corresponding increase in hydrogen consumption. Additionally, in the vicinity of the high-temperature section the economically more beneficial gas heating must be dispensed with in flavor of more expensive electrical heating.
The object of the present invention therefore is to create a tower furnace of the type outlined initially for heat treatment of metal strips, such that despite the restrictions imposed by the heat and weight loads of the muffle the required final strip temperature can be achieved without the disadvantages associated with the use of a high-temperature section without a muffle having to be considered.
The invention solves the task by the high-temperature section also being provided with a preferably gas-heated muffle which encloses the connecting stopper externally and connects to this preferably by way of a fluid seal in a gas-tight manner.
Since as a result of these measures the high-temperature section also has a muffle the advantages of a muffle can be exploited beneficially in the high-temperature section, particularly with respect to rinsing and possible gas heating. Due to restriction of the length of the muffle in the high-temperature section the muffle load can be limited by the dead weight, leading to a higher temperature capacity on account of the associated lesser stability requirements, which ensures the required end temperature of the metal strips to be treated. Only a transfer as heat loss-free as possible between the pre-heating section and the high-temperature section by way of the connecting stopper is to be provided. For this purpose the connecting stopper is enclosed externally by the lower end of the muffle of the high-temperature section, whereby a gas-tight connection must be attained between connecting stopper and muffle of the high-temperature section, and this with the possibility of axial displacement of the muffle compared to the connecting stopper for equalising heat expansion of the muffle. A fluid seal known per se can be used for this purpose to advantage. But it is also possible to produce the gas-tight connection by using a bellows-like sleeve positioned between muffle and connecting stopper. The heat-insulating fireproof material of the connecting stopper has a relatively small volume, such that the rinsing procedure is barely influenced by the connecting stopper. Since the connecting stopper is located in the vicinity of the thermal radiation and the muffle of the pre-heating section as well as the muffle of the high-temperature section, strip heating substantially covering the length of the furnace can be assumed having an advantageous effect on the required overall length of the furnace.
For ensuring strip heating by the connecting stopper which is as unhindered as possible the connecting stopper should be provided with a sufficiently large through cross-section for the metal strips so that the heat radiation of both muffles can extend into the region of the connecting stopper also. For this very purpose the connecting stopper can have a through cross-section for the metal strip to be treated with a minimum width measured vertically to the metal strip and corresponding to half the external radius of the connecting stopper.